1. Field of the Invention
This invention generally relates to a CRT display device and in particular to a display control system of a raster-scanning type CRT display device for use in a system such as a word processor. More specifically, the present invention relates to a CRT display control system which allows all of the characters in a single print-out line to be displayed in a single row in the display area of a CRT screen by changing the pitch and the width size of some of the characters even if the number of the characters in a single line has increased.
2. Description of the Prior Art
The CRT display device has been widely used and has various applications, one of which is a word processor. As shown in FIG. 1, the word processor is generally comprised of a central processing unit (also referred to as CPU hereinbelow) 1, a keyboard 2 as an input device, a CRT 3 as an intermediate output device, a printer 4 as an output device and a file device 5 as the storage for information. In this system, word information such as letters, numbers and figures is introduced by the operator from the keyboard 2 and temporarily stored in the file device 5 according to the instructions fed from the CPU 1. The information thus introduced may be displayed on the screen of the CRT 3 for confirming and/or correcting the information before supplying the information to the printer 4. Also provided is a control unit 6 which interacts with each of the keyboard 2, CRT 3, printer 4 and file device 5, and thus the control unit 6 has an interfacing function.
For a CRT display device in which use is made of the dot matrix in displaying characters on the screen, it has been common practice to display a predetermined number of characters in a line with a fixed dot pitch as driven by a reference clock having a predetermined frequency. In such prior art CRT display devices, there was a limit in the number of characters to be displayed in one line or row of the display area. Accordingly, if the maximum or predetermined number of characters to be displayed in one line is 40, then those characters beyond 40 cannot be displayed in the same line at the same time.
FIG. 2 schematically shows the display area 7 on the screen of the CRT device 3. For the sake of simplicity, only representative characters 8 are shown in FIG. 2. It should, however, be understood that the display area 7 of FIG. 2 has a maximum capacity of displaying 960 characters. That is, the display area 7 has 40 columns in the horizontal or X direction and 24 rows in the vertical or Y direction. The characters 8 are displayed as equally spaced from each other in a line with a constant dot pitch in this example. The arrow A indicates the scanning direction across the area 7.
FIG. 3 shows one example of the character 8 which represents the English letter "A" as displayed by the dot matrix which includes 63 dots arranged in 7 columns D.sub.1 through D.sub.7 and 9 rows L.sub.1 through L.sub.9. In FIG. 3, the blackened dots are illuminated dots; whereas, the white dots are non-illuminated or blank dots, thereby the letter "A" is represented by the blackened or illuminated dots.
FIG. 4 shows in block diagram the architecture of the conventional control unit 6. An address signal ADR in the form of a code corresponding to a particular character to be displayed is supplied from the CPU 1 via the CPU bus 11. The ADR signal is then supplied to a refresh memory 14 and stored temporarily therein after passing through a column address selector 12 and a row address selector 13.
When clock signals generated by a timing pulse generator 22 are supplied to a column address counter 17, the counter 17 starts counting and its contents at the termination of the counting operation forms a column address signal to be supplied to the refresh memory 14 through the column address selector 12. The output from the column address selector 17 is also supplied to a horizontal synchronizing pulse generator 20 and to a line counter 19. The horizontal synchronizing pulse generator 20 supplies a horizontal synchronizing signal HS as its output, and the line counter 19 supplies a line signal as an input to a character generator 15. The output of the line counter 19 is connected to the input of a row address counter 18 which counts to form a row address of the refresh memory 14 upon completion of counting the one line of a character by the line counter 19. The output signal from the row address counter 18 is supplied to the refresh memory 14 via the row address selector 13; on the other hand, it is also supplied to a vertical synchronizing pulse generator 21 which supplies a vertical synchronizing signal VS as its output.
A particular cell of the refresh memory 14 is addressed by the column address and row address signals thus supplied, and a particular code thus addressed is supplied as an address signal to the character generator 15 for reading out the dot pattern information of a desired character. The dot pattern information is then supplied to a parallel-in-series-out shift register 16 to produce a video signal VI to be supplied to the CRT 3. In such prior art CRT display devices, since locations of displaying characters are predetermined in the display area on the screen of the CRT with a constant pitch or distance between the two adjacent characters, there is a maximum limit in the number of characters to be displayed in the display area. For example, if the display area is designed for 40 columns and 24 rows and a chain of characters to be displayed consists of 41 or more characters, then all of the characters cannot be displayed in a single row at the same time in prior art CRT display devices.
It is often desired that a greater number of characters be displayed in the display area at the same time as the volume of information to be handled increases. For example, in a system such as shown in FIG. 1, the printer 4 usually has an ability to print out a greater number of characters in a single row line on a print-out paper as compared with the number of characters to be displayed in a row line of the display area of CRT device 3. One example of such a print-out paper 9 is shown in FIG. 5 and 60 characters at maximum may be printed out in a single row in this example, which is far greater than the maximum number of characters, which is 40, to be displayed in a single row in the display area 7 of the CRT device 3 as shown in FIG. 2. In this case, 20 of 60 characters to be printed out in a single row cannot be shown in the display area 7, which is quite inconvenient for the operator.
It is true that use may be made of a large-sized display area which is capable of displaying characters in the same manner as they are printed out. However, this is not advantageous because manufacturing a large-sized CRT can be quite expensive. Alternatively, all of the characters may be reduced in size to be fully displayed in the display area 7. This is also disadvantageous because smaller-sized characters are hard to see and frequent change in the size of characters will easily tire the operator.
Under the circumstances, there were three conventional approaches to cope with this problem. First approach was to divide the total column number of characters to be printed out, e.g., 60 in the above example, into two parts and to display these in sequence one after the other. That is, the first 40 columns of characters are displayed at one time and subsequent thereto the remaining 20 columns of characters are displayed. Second approach was to divide the total number of characters to be printed out in a single row into two parts and to display them in two consecutive rows as shown in FIG. 6. Third approach was to project characters toward the display area 7 in the same format as they are printed out with the excessive ones projected to the right or left of the area 7 as shown in FIG. 7 thereby the excessive ones do not fall onto the display area. In the example shown in FIG. 7, 60 columns of characters are projected but only 40 of them are displayed in the display area 7 at the same time. The character columns are then shifted to the left or right as appropriate to display all of the columns of characters in sequence. This third approach is called the scroll display method.
However, none of the above-described conventional approaches is satisfactory. For example, in the second approach, one print-out line requires two rows in the display area, which is quite disadvantageous since the display capacity is not fully used. Moreover, this approach requires a complicated display control unit and the characters displayed are rather difficult to observe. On the other hand, the first and second approachs are disadvantageous in that all of the columns of characters are not displayed at the same time so that they are dependent upon the ability of the operator to memorize what he or she has seen during operation.
Particularly for a system like a word processor, the editing function to change the arrangement of letters and words is very important. In this instance, it is quite inconvenient if all of the characters in a single row are not shown in the display area at the same time.